Doomguard Roulette in Ironforge's AH or goldshire was probably my favorite. Fear was pretty funny before it got diminishing returns. watching rogues run around helplessly as dots slowly killed them was one of my favorite pass times. Summoning people off cliffs, under water, etc. lets see... Oh! find a lowbie that was struggling a bit, then use Fear on their target to cause it to run into a pack and curse of recklessness when they aggroed. bonus points if you could convince them you were helping.

Level 70 undead in TBC, wandered my way over to the Human starting zone after decimating Goldshire, proceed to use max level Curse of recklessness on wolves and kobolds newb allies were trying to kill. Priceless.

Also intentionally pulling more threat than the tank during Prince P2, then as soon as he switched over to me popped soulshatter so the next highest dps(fire destro lock friend of mine) ended up with the aggro. Everyone blamed him for the wipe because he was the first to die, he tried in vain to explain to the Pug group that I had soulshattered away my threat, but back then no one knew how locks worked aside from shadow bolt so he god kicked.

Running into an alliance world boss raid and popping howl of terror in the middle of their healers and then Death Coil'ing the tank is always fun.

Intentionally stacking more than one soulstone on someone is something I enjoy to this day.

Bak in vanilla, when warlocks could not conjure the summoning closet thing, if you positioned yourself really really close to the edge of a cliff you could summon people right off the edge. When people were late for our Molten Core runs, the locks in our guild would summon people directly over the lava pit that's below the crystal that teleported you in.

It was easy to be ebil in vanilla so I did most of it back then including but not limited too.

-Click the table for epix
-Summoning people underwater to drown(most peeps would almost be dead before they even loaded into the area)
-Summoning people off cliffs/into lava

But, my masterpiece of EBIL was in BWL, back then the imp was phase-shifted until he attacked something so he wouldn't aggro anything until he popped out. I was really really sick of the mages one night being all uppity about curse of elements not being up fast enough for thier liking so I /tar chromaggus and sent my imp to attack him(back then the imp would attack chromie through his gate) I pulled chromaggus right after we killed Broodlord so Chromie come trucking on down into the raid with every mob between his room and us and he brings ebonroc, flamgor, and firemaw to the party, needless to say everyone died.

Best part was when the raid leader asked WTF just happened the rogue class leader said it just happened sometimes if someone clipped through the wall during the broodlord fight.

Welcoming the new priest healer to our guild: http://www.youtube.com/watch?v=PGMEXO3aO4w
She made the mistake of asking if her soul was part of the requirements for joining the guild, and I had to explain that her sacrifice is not for the guild, nor cookies, but my personal amusement. So we made this happen. Sadly, she didn't have a sacrificial dagger to die with.

I used to use have group will travel to summon large groups of people to random places. Such as the middle of stormwind trade district, bottom of large bodies of water, on top of really tall things that they can all fall off of. and to large rocks in the middle of outland.

edit: also since we're all warlocks at heart even when not on them I also like to get on my mage and setting up portals on top of warlock summoning stones. People like to click on them thinking theyre' summoning someone

Blizzard, HOW DARE YOU! How dare you spend precious development time making a new, interesting feature and then turn around and try to make money off said development time. I AM ENTITLED TO EVERYTHING YOU EVER MAKE FOR FREE BECAUSE I BOUGHT THAT GAME THAT ONE TIME.

Credit goes to Chris Hanel of The Daily Blink for the beautiful avatar picture.

3rd (or 4th?) boss in hyjal casting rain of fire on people I didn't like just to see them run since they thought it was the boss mechanic...(once it was on farm). More recently setting up gateways on ToT windy walkway after jinrok that fedex'd them into the abyss. Doomguard roulette was always fun back in old days. Still recall the first kill of vasj where my curse of doom tick killed an add and thereby summoned a doomguard mid fight...and we still got our first kill!